Tidal Layers Show Days Were Shorter 900 Million Years Ago

University of Arizona scientists counting the microscopic stripes in sedimentary rocks--a record left by the rise and fall of daily tides--have determined that days really are getting much longer.

At least 900 million years ago, the length of a day on Earth was only about 18 hours and 10 minutes, compared to today's 24 hours, they determined. Tidal layers in ancient rocks can be used like tree rings to measure such time intervals very precisely.

The measurements, reported in Science, also suggest that the moon then was much closer to Earth than it is today and that Earth took much longer to complete a single orbit around the sun.